A leading health expert has told the ABC that a wider evacuation may be wise if the fire continues.

Queensland University of Technology professor Lidia Morawska is an expert on how air quality affects human health and says the situation in Morwell is serious.

"There's been cases like this studied and studied for many decades now showing that exposure of a duration of a few days or two weeks can lead to mortality," she said.

"So you can't have a longer-term effect than this. There are definitely long term effects possible from this duration of exposure related to cardiac problems and respiratory problems."

Professor Morawska says understanding the risks of exposure in relation to cancer is more complicated.

"Cancer develops much later after the exposure and whether an exposure of two weeks is sufficient to develop cancer - I don't think there is a study which would be able to point out to this," she said.

"But having said this we can't exclude this."

Given the uncertainty about the likely health effects, Professor Morawska says she would suggest people err on the side of caution and try to leave the area.

Government defends Dr Lester's advice

Premier Denis Napthine says he has full confidence in Dr Lester's advice, and that all of the Government's decisions have been guided by experts.

"I as Premier of this state have enormous confidence in Dr Rosemary Lester," he said.